Used named constants everywhere

[stefan6419846]

At the moment, we are not using named constants consequently, which does not feel correct.

This is a meta-issue to track the aspects to consider for proper usage.

• Always import the classes by their full name instead of using aliases.
• Use existing constants where available.
• Add new constants where missing.
• Re-organize constants.py, maybe by making it a module with submodules.
• Simplify usage of constants by providing an easy way to get a PdfObject (for example NameObject for names) directly without having to think about the explicit conversion.